Why are my bones bendy? Rigging
I think doing what's described here How do rigs relate to weights? will help.

If you parent the mesh to the bones 'with automatic weights', it should try to correctly weight paint the mesh to the bones fairly well. And if still looks 'weird' on some parts such as 'too sharp' bending, or 'too soft' (like what you're experiencing), then you need to adjust the weight painting a bit.

hope it helps!